METHOD AND DEVICE FOR SEPARATING HEAVY PARTICLES FROM A PARTICULATE MATERIAL

Abstract: In a method for separating heavy particles from a particulate material containing such heavy particles, use is made of a device having a Coanda sifter, which is adapted to receive a flow of the particulate material and which has a downwardly-facing, convexly single-curved surface (9), and a zigzag sifter (13) arranged after the Coanda sifter and having a plurality of zigzag-shaped, vertical walls (14) which are perpendicular to the axis of curvature of the single-curved surface (9) and define between them channels (15) extending upwards in zigzag fashion. The particulate material is transported to the Coanda sifter and is conducted over its convexly single-curved surface (9) to have imparted to it a direction of motion (P1, P2) which is dependent on the weight of the particles and in which the particles are guided into the zigzag sifter (13). The particles are guided into the channels (15) of the zigzag sifter (13) in the lower portions thereof and from the side parallel to the channel walls (14). The particles are transported up through the channels (15) with the aid of a flow of transport air which is so adjusted that the heavy particles are separated in the zigzag sifter (13).

Description: METHOD AND DEVICE FOR SEPARATING HEAVY PARTICLES FROM A PARTICULATE MATERIAL The present invention relates to a method for sepa¬ rating heavy particles, such as latex particles, from a particulate material, such as wood fibres produced from rubber trees, which contains such heavy particles and which is transported by means of air from a first sta¬ tion, such as a container, to a second station, such as a plant for manufacturing fibreboards. The invention also relates to a device for carrying out the method. In the manufacture of fibreboards, wood fibres are pneumatically transported, first from a drying plant to a collecting container and thence to a forming station. In the forming station, the fibres are placed as a mat on a wire, through which the transport air is sucked off. The fibre mat is thereafter pressed into fibreboards under given pressure and temperature conditions. In the manufacture of fibreboards, it is essential that contaminants in the form of heavy particles be sepa¬ rated from the flow of material. In a method, known from PCT/SE91/00603 (SE 9003092-5), for separating large-size and hence heavy particles from the wood fibres when transported from the collecting container to the forming station, the wood fibres are introduced into a settling chamber as a downwardly-directed flow of material via a downwardly-facing settling chamber inlet, and is trans- ported away from the settling chamber as an upwardly- directed flow of air-material via a settling chamber out¬ let disposed beside the inlet. Jets of compressed air, directed obliquely at the downwardly-directed flow of ma¬ terial, are blown into the settling chamber in order to decelerate the flow of material and to spread it in the transverse direction towards the outlet, the flow of transport air being blown into the settling chamber so as to encounter from below the decelerated and spread flow of material and, together with it, form the above-men¬ tioned flow of air-material. While this known method has proved to serve its pur- pose quite well for separating heavy particles in the form of glue lumps and fibre flocks from a particulate material in the form of glue-coated wood fibres, it yields an unsatisfactory result when separating heavy particles in the form of latex particles from a particu- late material in the form of wood fibres produced from rubber trees. One object of the present invention is to provide a method making it possible to efficiently separate also latex particles from wood fibres produced from rubber trees. According to the invention, this object is achieved by a method which is of the type stated by way of intro¬ duction and which is characterised in that the particulate material in a substantially downwardly-directed flow is transported in known manner to a so-called Coanda sifter, in which the material is conducted over a downwardly-fac¬ ing, convexly single-curved surface in order to impart to the particles a direction of motion which is dependent on the weight of the particles and in which the particles are guided into a so-called zigzag sifter disposed after the Coanda sifter and having a plurality of zigzag-shaped, substantially vertical walls which define between them channels extending upwards in zigzag fashion, the par¬ ticles being guided into the channels of the zigzag sifter in the lower portions thereof and from the side substan¬ tially parallel to the walls, and that the particles are transported in known manner up through the zigzag-shaped channels by means of a flow of transport air which is so adjusted that the heavy particles are separated in the zigzag sifter. Another object of the present invention is to pro¬ vide a device for carrying out this method. According to the invention, this object is achieved by means of a device which is characterised by a so-cal¬ led Coanda sifter which is adapted in known manner to receive a substantially downwardly-directed flow of the particulate material and which has a downwardly-facing, convexly single-curved surface in order to impart to the particles a direction of motion dependent on the weight of the particles, and a so-called zigzag sifter disposed after the Coanda sifter and having a plurality of zigzag- shaped, substantially vertical walls which are substan¬ tially perpendicular to the axis of curvature of the single-curved surface and define between them channels which extend upwards in zigzag fashion, said zigzag sifter being adapted to receive the particles coming from the Coanda sifter, in its lower portion and from the side. The folds defining the zigzag shape of the walls are inclined preferably 5 c -20°, especially 15°, downwards in relation to the horizontal plane in a direction away from the Coanda sifter. The invention will now be described in more detail with reference to the accompanying drawings. Fig. 1 schematically shows a device for separating heavy particles from a particulate material. Fig. 2 shows a zigzag sifter included in the device of Fig. 1 in the direction of the arrow II in Fig. 1. The device schematically illustrated in the drawings is arranged between a container 1, which holds wood fibres derived from rubber trees and containing contami- nants in the form of heavy particles, such as latex par¬ ticles, and a forming station (not shown), where fibre- boards are formed. The bottom of the container 1 consists of the upper run of an endless conveyor belt 2, on which a heap 3 of wood fibres is supported. A plurality of rotatable slush¬ ing rollers 4, provided with spikes or teeth, are arrang¬ ed in the front part of the container 1 in order, as the conveyor belt 2 is discharging wood fibres from the con¬ tainer 1, to break up fibre flocks and lumps which have formed when storing the fibres in the container 1. The container 1 has an outlet 5 which is located at the exit end of the conveyor belt 2 and communicates with a substantially vertical discharge shaft 6. The container 1 has a front wall 7 forming an extension of the front wall 8 of the shaft 6 and having an air intake 7' pro¬ vided therein. The front wall 8 of the shaft 6 passes at its lower portion into a downwardly-facing, convexly single-curved surface 9 which in the illustrated embodiment consists of the lower half of the outer circumferential surface of a horizontal, straight, circular cylinder. The surface 9 forms a so-called Coanda sifter known in the art, i.e. a sifter relying on the Coanda effect. A sifter of this type is described in more detail in Swedish Patent Speci¬ fication 8505726-3. The rear wall 10 of the shaft 6 has in its lower portion, on a level with the Coanda sifter, an inlet opening 11. A fan 12 is adapted, via the opening 11, to blow a high-velocity air flow directed substan¬ tially at the surface 9 into the lower part of the shaft 6. The wood fibres fed from the container 1 into the shaft 6 by means of the conveyor belt 2 are transported with the aid of the air flow towards the Coanda sifter. As a result of the Coanda effect, the air flow will fol¬ low the curved surface 9 and entrain the wood fibres (arrows PI in Fig. 1), while the heavy particles are not deflected to the same extent, but follow a straighter path (arrows P2 in Fig. 1) . A so-called zigzag sifter 13 known per se is arranged immediately after the Coanda sifter for separat¬ ing the heavy particles (latex particles, glue lumps etc). A zigzag sifter of this type is described in German Offenlegungsschrift 1,482,424. The zigzag sifter 13 has a plurality of zigzag-shaped, vertical metal sheets 14 (Fig. 2), which are perpendicular to the axis of curva¬ ture of the surface 9 and define between them channels 15 extending upwards in zigzag fashion. As shown in Fig. 1, the wood fibres (arrows PI) and the heavy particles (arrows P2) leaving the Coanda sifter are guided into the channels 15 of the zigzag sifter 13 in the lower portions thereof and from the side, i.e. parallel to the sheets 14. Then, the wood fibres and the heavy particles are distributed when entering the zigzag sifter 13 across the width of the channels 15, such that the heavy particles will enter deeper (to the left in Fig. 1) than the wood fibres. An additional distribution of the wood fibres and the heavy particles, resulting in a deflection of the heavy particles to the left (Fig. 1) during the operation of the zigzag sifter 13, is obtained by the zigzag sifter being so inclined that the folds 14' defining the zigzag shape of the sheets 14 are inclined 5°-20° downwards in relation to the horizontal plane in a direction away from the Coanda sifter. In the illustrated embodiment, this inclination is about 15°. A conveyor screw 16 is arranged in the lower, left- hand part (Fig. 1) of the zigzag sifter 13 for removing the heavy particles separated in the zigzag sifter 13. An air supply conduit 17 opens in the lower, right-hand part (Fig. 1) of the zigzag sifter 13. A fan 18 is provided, via the conduit 17, to blow a flow of transport air into the zigzag sifter 13. This flow transports the wood fibres through the zigzag sifter while the heavy par¬ ticles (latex particles) are separated therein and drop on to the conveyor screw 16. The zigzag sifter 13 has an upper outlet 19 having an upwardly decreasing cross-sectional area and communi¬ cating with a transport conduit 20 for transporting the wood fibres freed from heavy particles to the forming station. In its upper portion, the transport conduit 20 has a 180° bend, after which it is divided into two sepa¬ rate conduits 20a and 20b, of which the conduit 20a passes on to the forming station and the conduit 20b returns to the zigzag sifter 13 and opens at the mouth of the conduit 17. Guide baffles 21 are arranged in the lower part of the conduit 20b. The flow of air-material consisting of transport air and wood fibres is accelerated in the outlet 19 as a result of the decreasing cross-sectional area thereof. The flow then has a high velocity in the bend of the transport conduit 20, causing the wood fibres therein to be flung outwards so as to follow the outer wall of the bend in order, together with a portion of the transport air, to be guided into the conduit 20a, leading to the forming station. The rest of the transport air is guided into the conduit 20b and recycled to the zigzag sifter. The device described above yields a good result also in the separation of latex particles from wood fibres derived from rubber trees. This is assumed in particular to depend on the distribution of the particles across the width of the channels 15 which is brought about with the aid of the Coanda sifter and to which the inclination of the zigzag sifter 13 also contributes. Since the particles are spread over the width of the zigzag sifter 13, such that the particles will enter deeper into the sifter (i.e. farther to the left in Fig. 1) the heavier they are, the wood fibres freed from heavy particles may, if so desired, easily be divided, for example into two fractions. This is done by dividing the upper outlet 19 of the zigzag sifter 13 into two separate outlets, one disposed over the upper, left-hand part (Fig. 1) of the zigzag sifter 13, and the other over the upper, right-hand part (Fig. 1) thereof.

Claims: CLAIMS 1. A method for separating heavy particles, such as latex particles, from a particulate material, such as wood fibres produced from rubber trees, which contains such heavy particles and which is transported by means of air from a first station, such as a container (1), to a second station, such as a plant for manufacturing fibre- boards, c h a r a c t e r i s e d in that the particulate material in a substantially downwardly-directed flow is transported in known manner to a so-called Coanda sifter, in which the material is conducted over a downwardly-fac¬ ing, convexly single-curved surface (9) in order to impart to the particles a direction of motion (PI, P2) which is dependent on the weight of the particles and in which the particles are guided into a so-called zigzag sifter (13) disposed after the Coanda sifter and having a plurality of zigzag-shaped, substantially vertical walls (14) which define between them channels (15) extending upwards in zigzag fashion, the particles being guided into the channels of the zigzag sifter in the lower por¬ tions thereof and from the side substantially parallel to the walls, and that the particles are transported in known manner up through the zigzag-shaped channels (15) by means of a flow of transport air which is so adjusted that the heavy particles are separated in the zigzag sifter (13). 2. A device for carrying out the method as claimed in claim 1 for separating heavy particles, such as latex particles, from a particulate material, such as wood fibres produced from rubber trees, which contains such heavy particles and which is transported by means of air from a first station, such as a container ( 1 ) , to a second station, such as a plant for manufacturing fibre- boards, c h a r a c t e r i s e d by a so-called Coanda sifter which is adapted in known manner to receive a substantially downwardly-directed flow of the particulate material and which has a downwardly-facing, convexly single-curved surface (9) in order to impart to the par¬ ticles a direction of motion (PI, P2) dependent on the weight of the particles, and a so-called zigzag sifter (13) disposed after the Coanda sifter and having a plu¬ rality of zigzag-shaped, substantially vertical walls (14) which are substantially perpendicular to the axis of curvature of the single-curved surface (9) and define between them channels (15) which extend upwards in zigzag fashion, said zigzag sifter being adapted to receive the particles coming from the Coanda sifter, in its lower portion and from the side. 3. A device claimed in claim 2, ch a r a c t e r - i s e d in that the folds (14' ) defining the zigzag shape of the walls (14) are inclined 5°-20°, especially 15°, downwards in relation to the horizontal plane in a direc¬ tion away from the Coanda sifter.

Inventors: GUSTAVSSON LENNART, GUSTAVSSON, LENNART
Applicants: FLAEKT AB, GUSTAVSSON LENNART, ABB FLAEKT AB, GUSTAVSSON, LENNART
Application year: 1994
Main classification: B07B7
IPC codes: B07B7, B07B9